FIR checker: differentiate UNSAFE_CALL from INAPPLICABLE_CANDIDATE

To do so, inside the root cause of inapplicable candidate errors,
we will record expected/actual type of receiver, if any.
That will help identifying inapplicable calls on nullable receiver.

compiler/fir/analysis-tests/testData/resolve/smartcasts/boundSmartcasts/boundSmartcastsInBranches.kt

@@ -104,18 +104,18 @@ fun test_7() {
 
     if (x != null) {
         x.length // OK
-        y.<!INAPPLICABLE_CANDIDATE!>length<!> // Bad
+        y.<!UNSAFE_CALL!>length<!> // Bad
         z.length // OK
     }
     if (y != null) {
-        x.<!INAPPLICABLE_CANDIDATE!>length<!> // Bad
+        x.<!UNSAFE_CALL!>length<!> // Bad
         y.length // OK
-        z.<!INAPPLICABLE_CANDIDATE!>length<!> // Bad
+        z.<!UNSAFE_CALL!>length<!> // Bad
     }
 
     if (z != null) {
         x.length // OK
-        y.<!INAPPLICABLE_CANDIDATE!>length<!> // Bad
+        y.<!UNSAFE_CALL!>length<!> // Bad
         z.length // OK
     }
 }